Bug-for-bug compatibility [was RE: SimH DECtape vs. Tops-10 [was RE: Writing emulators [Was: Re: VCF PNW 2018: Pictures!]]]

[David Bridgham]

> Imagine our chagrin when days of trying to correct the
> problem led to the conclusion that the diagnostic was incorrect.

I may have a situation like this in working on my FPGA PDP-10.  The
Processor Reference Manuals seem quite clear that the rotate
instructions take E mod 256.  One of the manuals I've found even adds
that they never move more than 255 positions.  And yet the diagnostics I
have clearly want ROT AC,-256 to move 256 positions to the right, not
0.  Not having a real PDP-10 to compare against, I don't know which is
right.

Doing it mod 256 would be easier; I had to add an extra rotor to my
barrel shifter to handle the -256 case to make the diagnostic pass.